David,

Thanks!!

I had the same issue, today, and worked fine.

Ok, I thought your question was the one I answered. Here are some examples of what I think you are asking based on my JSON file:

In[31]:= "required" /. jsonData

Out[31]= {"firstName", "lastName"}

In[33]:= "properties" /. jsonData

Out[33]= {"firstName" -> {"type" -> "string"}, 
 "lastName" -> {"type" -> "string"}, 
 "age" -> {"description" -> "Age in years", "minimum" -> 0, 
   "type" -> "integer"}}

In[34]:= "firstName" /. ("properties" /. jsonData)

Out[34]= {"type" -> "string"}

In[35]:= "type" /. ("firstName" /. ("properties" /. jsonData))

Out[35]= "string"

I put the following JSON file on my computer in a file called test.json

{
    "title": "Example Schema",
    "type": "object",
    "properties": {
       "firstName": {
         "type": "string"
       },
       "lastName": {
         "type": "string"
       },
       "age": {
         "description": "Age in years",
         "type": "integer",
         "minimum": 0
       }
    },
    "required": ["firstName", "lastName"]
}

When I import it I get the following:

jsonData=Import["/Users/dreiss/Desktop/test.json"]

which gives the following:

{"required" -> {"firstName", "lastName"}, "title" -> "Example Schema",
  "type" -> "object", 
 "properties" -> {"firstName" -> {"type" -> "string"}, 
   "lastName" -> {"type" -> "string"}, 
   "age" -> {"description" -> "Age in years", "minimum" -> 0, 
     "type" -> "integer"}}}

Note that this is somewhat more nested than your example. If you want the left hand sides of the rules at the very top level just execute

First/@jsonData

which returns

{"required", "title", "type", "properties"}

Let's say you want all of the left hand sides of all rules then this would do the trick:

Cases[jsonData, (z_ -> _) :> z, Infinity]

which returns

{"required", "title", "type", "type", "firstName", "type", \
"lastName", "description", "minimum", "type", "age", "properties"}

By the way leaving out the Infinity, gives the same answer as the original computation which asks for just the first level:

Cases[jsonData, (z_ -> _) :> z]

returns

{"required", "title", "type", "properties"}

Let's say though though you want only the left hand sides all the way to only the third level

Cases[jsonData, (z_ -> _) :> z, 3]

gives

{"required", "title", "type", "firstName", "lastName", "age", \
"properties"}

And if you want it only at just the 3rd level you'd use this

Cases[jsonData, (z_ -> _) :> z, {3}]

which gives this:

{"firstName", "lastName", "age"}